The structural protein tubulin is involved in a variety of important processes in morphogenesis, including cell division, cell shape and flagellar motility. The assembly of tubulin into microtubule arrays with different functions may utilize discrete structural domains of the tubulin molecule, and may be mediated by interactions between tubulin and different sets of microtubule associated proteins. Our long term goal is to elucidate both the cis and trans-acting mechanisms that regulate the assembly and function of architecturally different microtubule arrays in vivo. We have identified a network of interacting mutations involved im microtubule function that fail to complement certain recessive alleles of the testis-specific Beta tubulin gene of Drosophila (B2t) but that map to other genes. These second-site non-complementing mutations (nc) may identify genes that encode proteins that interact with Beta tubulin at the structural level. To deduce the role of these interacting genes in microtubule function in vivo, we will examine the effects of mutant alleles on morphological changes that are mediated by different kinds of microtubule arrays. To determine the molecular basis for this unusual genetic interaction, we will identify the protein products encoded by selected nc genes and analyze their interactions with tubulin at the biochemical level. We will identify DNA sequences that contain nc genes by transformation and genetic assays, express the nc gene products, and raise antibodies to use for identification and localization of the nc gene products in specific microtubule arrays. In addition, we have identified mutations in the Beta2 tubulin structural gene that encode partially functional tubulin subunits, and cause different defects in microtubule function during spermatogenesis. These mutations may define functional domains of the Beta2 tubulin molecule and allow us to correlate specific defects observed in vivo with changes in biochemical properties of the mutant subunits, such as the ability to bind specific microtubule associated proteins. Finally, we will extend our genetic analysis of microtubule function to mitosis by analysis of the genetic relationships between the mutations that interact with Beta2 tubulin and a set of mutations that fail to complement the mitotic mutant dsp.